Semiconductor integrated circuit device

ABSTRACT

A memory cell of the type a pair of cross-coupled CMOS inverters of a SRAM is disclosed in which the load MISFETs are stacked above the semiconductor substrate and over the drive MISFETS. Each load MISFET of a memory cell consists of a source, drain and channel region formed within the same polycrystalline silicon film, and a gate electrode consisting of a different layer conductive film than that of the drive MISFETs. In a memory cell having such a stacked arrangement, the source (drain) region and gate electrode of each load MISFET thereof are patterned to have an overlapping relationship with each other so as to increase the effective capacitance associated with each of the memory cell storage nodes. The gate electrodes of both the drive and load MISFETs are formed of n-type and p-type polycrystalline silicon films, respectively, and the drain regions of the first and second p-channel load MISFETs are electrically connected to the drain regions of the first and second n-channel drive MISFETs through separate polycrystalline silicon films, respectively. The polycrystalline silicon gate electrodes of the first and second load MISFETs are respectively electrically connected to the drain regions of the second and first drive MISFETs in each memory cell of the SRAM, furthermore.

This is a divisional of application Ser. No. 08/028,128 filed Mar. 9, 1993, now U.S. Pat. No. 5,483,083, which is a divisional of application Ser. No. 07/837,689 filed Feb. 19, 1992, now U.S. Pat. No. 5,194,749; which is a continuation of application Ser. No. 07/625,682 filed Dec. 12, 1990, now abandoned; which, in turn, is a continuation of application Ser. No. 07/274,490 filed Nov. 22, 1988, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor integrated circuit device and, more particularly, to effective technology to be applied to a semiconductor integrated circuit device having a SRAM (static random access memory) constituted by a memory cell CMOS type.

A SRAM memory cell of the CMOS type is composed of a flip-flop circuit comprising a pair of cross-connected (cross-coupled) CMOS inverter circuits each of which consisting of a series connection of an n-channel drive MISFET and a p-channel load MISFET, and a pair of n-channel transfer MISFETs each of which connecting a storage node of the flip-flop circuit to one of a pair of complementary data lines. The flip-flop circuit is supplied with the power source voltage Vcc and the ground potential, and a pair of data lines are respectively connected to the drain of the transfer MOSFETs wherein the common gate thereof becomes a word line. Operation of such memory cell of a known SRAM is effected in that the word line is raised and information (data) of "High" or "Low" from the data line is stored through the transfer MISFET to the storage node during the writing of data information stored in, or the storage node is read through the transfer MISFET during the reading of data, thereby the memory cell functions as a static memory. In such a SRAM memory cell of the CMOS circuit, since leak leakage current of the MISFET only flows through the memory cell at the waiting state, the consumption power is quite low.

In the SRAM memory cell of the CMOS type, however, since one memory cell is constituted by a total of six MISFETs, a problem occurs in that the chip size becomes large. An attempt to solve such problems is discussed in connection with a memory cell called "Stacked CMOS" described in "IEEE TRANSACTIONS ON ELECTRON DEVICES, vol. ED-32, No. 2, February 1983, p. 273-277". The memory cell called "Stacked CMOS" is constituted by p-channel load MISFET of a flip-flop circuit formed to a polysilicon film on n-channel drive MISFET. Further describing the memory cell using FIGS. 1 and 2, n-channel drive MISFET formed in a silicon substrate is converted at an upper portion and a side surface of a gate electrode 3b by at least a thin insulation film 14, and further a polysilicon film is provided on an upper portion and a side surface thereof. Source 5e, drain 5b and a channel member 5d of p-channel load MISFET are formed in the polysilicon film. Gate electrode of the p-channel load MISFET is common to the gate electrode 3b of the n-channel drive MISFET just below the channel member 5d, and a thin insulation film 14 becomes a gate insulation film of the p-channel MISFET. The drive MISFET of the flip-flop circuit is constituted by n-type impurity area 1e forming the common source, n-type impurity areas 1c, 1d forming drain and gate electrodes 3b, 3c. Respective gate electrodes 3b, 3c are connected through connection holes 2b, 2a to impurity areas at drain side in cross connection. N-type impurity areas 1c, 1d forming drain of respective drive MISFETs are common to source of the n-channel transfer MISFET connected to the flip-flop circuit and constitute storage node of the flip-flop circuit, and the transfer MISFET is constituted by a gate electrode 3a common to the source impurity area and n-type impurity areas 1a, 1b forming drain. The n-type impurity areas 1a, 1b are connected through connection holes 8a, 8b to aluminium electrodes 9a, 9b. The common gate electrode 3a constitutes a word line in the memory, and the aluminium electrodes 9a, 9b constitute data lines, respectively. On low-resistance polysilicon films 5a, 5b where the p-type impurity forming drain of the p-channel load MISFET is added at high density and gate electrodes 3b, 3c of the drive MISFET, connection holes 8e, 8f are opened so that respective areas are exposed commonly. Aluminium electrodes 9c, 9d connect between the polysilicon film 5a and the gate electrode 3b and between the polysilicon film 5a and the gate electrode 3c, respectively. The source of the p-channel load MISFET comprises a common low resistance polysilicon film 5e to which a p-type impurity is added at high density, and the power source voltage Vcc is supplied to the source of the two p-channel MISFETs. Channel members 5c, 5d of the p-channel MISFETs are arranged on the gate electrodes 3c, 3d of the drive MISFET respectively,

SUMMARY OF THE INVENTION

It has been found according to studying of the inventor that the above-mentioned memory cell has the following problems.

First, since the gate electrode of the n-channel drive MISFET formed in the silicon substrate and the gate electrode of the p-channel load MISFET in stacked state are common, the channel member of the p-channel load MISFET must be always arranged on the gate electrode of the drive MISFET. Consequently, degree of freedom becomes small in the case of the layout of the memory cell, thereby the area of the memory cell cannot be decreased efficiently.

Further, in order to form a thin insulation film on the gate electrode of the drive MISFET, material of the gate electrode may be limited. It is difficult to form a thin insulation film on a surface of a high melting-point metal such as tungsten or molybdenum which is required to accelerate the operation speed of the memory or on a surface of a silicide thereof, thereby such low resistance material cannot be used actually.

Also it has been reported until now that the drive capacity of the D-channel MISFET which is formed in a stacked state is small in comparison to that of the D-channel MISFET formed in the silicon substrate. For example, mobility of hole in the p-channel MISFET using the polysilicon is about 10 cm² /V.S. In the static memory having a load MISFET of such low drive capacity, the following problems arise. If α-rays generated during the decay of uranium (U) or thorium (Th), included in a small quantity in material, such as resin, to be used in the sealing of the memory chip or in a wiring material, such as aluminum are incident to the storage node portion in the "High" state within the memory cell, electron--positive hole pairs are generated along the range of α-rays and attracted by the electric field of the depletion layer so as to vary the potential of the storage node. As a result, if the potential variation has sufficient value to invert the flip-flop, information of the memory is destroyed. This is a phenomenon called soft error. In the memory cell of a SRAM of the CMOS type, in the prior art, where all MISFETs are formed in the silicon substrate, mobility of a hole indicating the drive capacity of the p-channel load MISFET is 200 cm² /VS or more thereby current can be supplied to the storage node following the potential variation of the storage node. In the memory cell of SRAM using the p-channel MISFET in stacked state, however, since the current drive capacity is small as above described, sufficient current cannot be supplied to the storage node before the information is destroyed because of the potential variation of the storage node.

Also charge of some degree is stored in the storage node portion due to capacitance of P-N junction formed in drain portion of the drive MISFET or gate capacitance, and there is no problem if the potential variation of the storage node can be restored by supply of this charge. In the memory cell in the high integrated state, however, since the area of the cell is small and therefore capacitance of the above-mentioned P-N junction or the gate capacitance is also small, the stored charge is small and further the current drive capacity of the p-channel MISFET is small, thereby sufficient charge cannot be supplied to the storage node. As a result, information of the memory cell is destroyed.

An object of the invention is to provide a SRAM having memory cell structure wherein degree of freedom in the case of the layout of the memory cell is large.

Another object of the invention is to provide a SRAM to enable the high speed operation.

Another object of the invention is to provide a SRAM having a memory cell which is resistant to soft error.

Accordingly, outline of typical improved aspects of the invention disclosed in the present application will be briefly described as follows:

A memory cell of SRAM of CMOS type has structure such that a load MISFET has a gate electrode which is formed separately from a gate electrode of a drive MISFET and disposed is on an upper side (above) of the drive MISFET.

A memory cell of SRAM of CMOS type has structure such that a gate electrode of a drive MISFET and a gate electrode of a transfer, which is MISFET formed simultaneously to the drive MISFET, have a silicide layer.

A memory cell of SRAM of CMOS type has structure such that a gate electrode of a load MISFET which is formed on the upper side of (or above a drive MISFET is overlapped with the source-drain area thereof thereby capacitance of a holding node is equivalently made large.

A memory cell of a SRAM of the CMOS type has a structure such that a load MISFET is formed on an upper side of (or above) a gate electrode of the drive MISFET thereof through an insulation film, wherein the load MISFET thereof has a gate electrode at lower side, and a channel area and source drain area position insulatedly on upper side of the gate electrode, thereby capacitance of a holding (storage) node is equivalently made large.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are a layout diagram and a main part sectional view of a SRAM memory cell in the prior art;

FIG. 3 is a main part sectional view of a SRAM memory cell of the invention;

FIG. 4 is a plan view of the memory cell;

FIG. 5 is an equivalent circuit diagram of the memory cell;

FIGS. 6 through 8 are plan views of the memory cell in the prescribed manufacturing processes;

FIGS. 9 through 15 are mainpart sectional views of the memory cell in each manufacturing process;

FIG. 16 is a sectional view illustrating a first modification of the invention;

FIGS. 17 through 19 are diagrams illustrating a second embodiment of the invention;

FIGS. 20 and 21 are sectional views illustrating the manufacturing method of the second embodiment of the invention;

FIGS. 22 and 23 are a sectional view and a plan view, respectively illustrating a third embodiment of the invention; and

FIGS. 24 and 25 are a plan view and a sectional view illustrating a fourth modification of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A memory cell of a SRAM as embodiment 1 of the present invention is shown in an equivalent circuit diagram of FIG. 5.

As shown in FIG. 5, the memory cell of SRAM is arranged at intersecting portion of complementary data lines DL, DL and a word line WL. The complementary data line DL extends in the column direction, and the word line WL extends in the row direction.

The memory cell is composed of a flip-flop circuit and two transfer MISFETs Qt₁ and Qt₂ with one semiconductor area (region) thereof connected to a pair of input/output terminals of the flip-flop circuit, respectively.

The transfer MISFETs Qt₁, Qt₂ are constituted by n-channel type, respectively. The other semiconductor area (region) of the transfer MISFETs Qt₁, Qt₂ is connected to the complementary data line DL, respectively. Each gate electrode of the transfer MISFETs Qt₁, Qt₂ is connected to the word line WL.

The flip-flop circuit is constituted as an information storage member (having information storage node portion). The flip-flop circuit is composed of two drive MISFETs Qd₁ and Qd₂ and two load MISFETs Qp₁ and Qp₂. The drive MISFETs Qd₁ and Qd₂ are constituted by n-channel type, and the load MISFETs Qp₁ and Qp₂ are constituted by p-channel type. That is, the flip-flop circuit is constituted by complete CMOS.

Respective source areas (regions) of the drive MISFETs Qd₁, Qd₂ are connected to the reference voltage Vss. The reference voltage is the ground potential 0 (V) of the circuit, for example. Drain area (region) of the drive MISFET Qd₁ is connected to the drain area (region) of the load MISFET Qp₁, one semiconductor area (region) of the transfer MISFET Qt₁, gate electrode of the drive MISFET Qd₂ and gate electrode of the load MISFET Qp₂. Drain area (region) of the drive MISFET Qd₂ is connected to the drain area (region) of the load MISFET Qp₂, one semiconductor area (region) of the transfer MISFET Qt₂, gate electrode of the drive MISFET Qd₁ and gate electrode of the load MISFET Qp₁. Respective source areas (region) of the load MISFETs Qp₁, Qp₂ are connected to the power source voltage Vcc. The power source voltage Vcc is the operation voltage 5 (V) of the circuit, for example.

Next, specific structure of the memory cell of SRAM constituted as above described will be briefly described referring to FIG. 4 (plan view) and FIG. 3 (sectional view taken in cutting line III--III of FIG. 4).

The memory cell, as shown in FIGS. 3 and 4, is provided on a main surface portion of p⁻ type well area 22 formed on a main surface portion of n⁻ type semiconductor substrate 21 of monocrystalline silicon. Although not shown. in an area other than the p⁻ type well area 22 n⁻ type well area is provided on main surface portion of the semiconductor substrate 21. Between memory cells or between elements each constituting a memory cell, a field insulation film 23 and p type channel stopper area 24 are provided on main surface of the well area 22. The field insulation film 23 and the channel stopper area 24 are constituted respectively so as to effect electrical isolation between memory cells or between elements each constituting a memory cell.

The transfer MISFETs Qt₁, Qt₂ of the memory cell respectively, as shown in FIGS. 3, 4 and 6 (plan view in the prescribed manufacturing process), are formed on a main surface of the well area 22 in an area surrounded by the field insulation film 23 and the channel stopper area 24. That is, the transfer MISFETs Qt₁, Qt₂ respectively are mainly composed of the well area 22, a gate insulation film 25, a gate electrode 27, a pair of n type semiconductor areas 29 being source area and drain area, and a pair of n⁺ type semiconductor areas (regions) 31.

The well area (region) 22 is used as a channel forming area.

The gate insulation film 25 is constituted by a silicon oxide film formed by oxidizing the main surface of the well area 22.

The gate electrode 27 of both the transfer MISFETs Qt₁, Qt₂ and that of the drive MISFETs Qd₁, Qd₂ is constituted in the same layer according to a prescribed upper portion of the gate insulation film 25. The gate electrode 27 is constituted by a composite film comprising a polycrystalline silicon film 27A and a high melting-point metal silicide film (WSi₂) 27B stacked on upper side of the silicon film 27A. The polycrystalline silicon film 27A is deposited by means of CVD, and n type impurity (P or As) to reduce the resistance value is introduced therein. The high melting-point metal silicide film 27B is deposited by means of sputtering or CVD. The gate electrode 27 constituted by the composite film has specific resistance value being small in comparison to that of a single layer of the polycrystalline silicon film, thereby the operation speed can be made high. Since the gate electrode of the transfer MISFETs Qt₁, Qt₂ is formed simultaneously with the gate electrode of the drive MISFETs Qd₁, Qd₂, i.e. the word line is formed using the high melting-point metal silicide film having small resistance value, high speed of read/write operation of information can be realized. Also since the gate electrode 27 has the upper layer constituted by the high melting-point metal silicide film 27B, irrespective of the conductivity type of the impurity introduced in the polycrystalline silicon film (34 and 37) of the upper layer of the gate electrode 27, the ohmic connection can be effected in connection to the polycrystalline silicon film of the upper layer.

Respective gate electrodes 27 of the transfer MISFETs Qt₁, Qt₂ are constituted integrally with the word line (WL) 27 extending in the row direction. The word line 27 is provided on the field insulation film 23.

The gate electrode 27 may be constituted by a composite film comprising the polycrystalline silicon film 27A and a high melting-point metal silicide film other than the above (MoSi2, TaSi2, TiSi2) or a high melting-point metal film (Mo, Ta, Ti, W) stacked on the silicon film 27A. Also the gate electrode 27 may be constituted by a single layer of the polycrystalline silicon film, the high melting-point metal film or the high melting-point metal silicide film.

A semiconductor area 29 of low impurity density is constituted integrally with a semiconductor area 31 of high impurity density, and provided at a side of the channel forming area in the main surface portion of the well area 22. The semiconductor area 29 of low impurity density constitutes the transfer MISFETs Qt₁, Qt₂ respectively in so-called LDD (Lightly Doped Drain) structure. The semiconductor area 29 of low impurity density is constituted in self-alignment to the gate electrode 27.

The semiconductor area 31 of high impurity density is constituted in self-alignment to a side wall spacer 30 formed on a side wall of the gate electrode 27.

The drive MISFETs Qd₁, Qd₂ of the memory cell respectively are formed in substantially the same manner as that of the transfer MISFETs Qt₁, Qt₂ respectively. That is, the drive MISFETs Qd₁, Qd₂ respectively are composed of the well area 22, the gate insulation film 25; the gate electrode 27, a pair of n type semiconductor areas 29 being source area and drain area, and a pair of n⁺ type semiconductor areas 31. The drive MISFETs Qd₁, Qd₂ respectively are constituted by the LDD structure.

One extending end of the gate electrode 27 of the drive MISFET Qd₂ passes through a connection hole 26, and is connected to one semiconductor area 31 of the transfer MISFET Qt₁ through the n⁺ type semiconductor area 28 interposed therein. Also one extending end of the gate electrode 27 of the drive MISFET Qd₁ passes through the connection hole 26, and is connected to one semiconductor area 31 of the transfer MISFET Qt₂ through the n⁺ type semiconductor area 28 interposed therein. The connection hole 26 is formed on the gate insulation film 25. The semiconductor area 28 is constituted by the n type impurity diffused from the polycrystalline silicon film 27A of the lower layer of the gate electrode 27 through the connection hole 26 onto the main surface portion of the well area 22.

The other extending end of the gate electrode 27 of the drive MISFET Qd₂ passes through the connection hole 26, and is connected to the semiconductor area 31 which is the drain area of the drive MISFET Qd₁ through the n⁺ type semiconductor area 28 interposed therein. The semiconductor area 31 being the drain area of the drive MISFET Qd₂ and one semiconductor area 31 of the transfer MISFET Qt₂ are constituted integrally.

A data line (DL) 40 is connected to respective other semiconductor areas 31 of the transfer MISFETs Qt₁, Qt₂ through a connection hole 39 formed on an interlayer insulation film 38. The data line 40 extends in the column direction on an upper side of the interlayer insulation film 38. The data line 40 is constituted, for example, by an aluminium film or an aluminium alloy film to which Cu or Si is added so as to prevent the migration.

The reference voltage Vss is applied to the semiconductor area 31 corresponding to the respective source areas of the drive MISFETs Qd₁, Qd₂. Supply of the reference voltage Vss is effected by reference voltage wiring which is formed by the same conduction layer as the gate electrode 27 and the word line 27, i.e., a composite film comprising the polycrystalline silicon film 27A and the high melting-point metal silicide film 27B, and extends in the same row direction. The reference voltage wiring is connected through the connection hole 26 formed on the gate insulation film 25 to the semiconductor area 31 corresponding to the respective source areas of the drive MISFETs Qd₁, Qd₂.

The load MISFET Qp₁ of the memory cell is constituted on an upper side of (i.e., insulatedly above) the drive MISFET Qd₂. The load MISFET Qp₂ is constituted on an upper side of (i.e. insulatedly above) the drive MISFET Qd₁. That is, respective load MISFETs Qp₁, Qp₂ are mainly composed of a gate electrode 34, a gate insulation film 35, a channel forming area 37A, a drain area 37B and a source area 37C.

As shown in FIG. 7 (plan view in the prescribed manufacturing process) in detail, the gate electrode 34 of the load MISFET Qp₁ is constituted on an upper side of the gate electrode 27 of the drive MISFET Qd₂ so as to cover it. The interlayer insulation film 32 is provided between the gate electrode 34 and the gate electrode 27. The gate electrode 34 of the load MISFET Qp₁ is connected through the connection hole 33 formed on the interlayer film 32 to the upper surface of the high melting-point metal silicide film 27B of the gate electrode 27 of the drive MISFET Qd₁. Consequently, the gate electrode 34 of the load MISFET Qp₁ is connected to the semiconductor area 31 corresponding to the drain area of the drive MISFET Qd₂ through the gate electrode 27 interposed therein. Also the gate electrode 34 of the load MISFET Qp₂ is constituted an upper side of the gate electrode 27 of the drive MISFET Qd₁ so as to cover it. The gate electrode 34 of the load MISFET Qp₂ is connected through the connection hole 33 to surface of the high melting-point metal silicide film 27B of gate electrode 27 of the drive MISFET Qd₂. Consequently, the gate electrode 34 of the load MISFET Qp₂ is connected to the semiconductor area 31 corresponding to the drain area of the drive MISFET Qd₁ which is integrally formed with one semiconductor area 31 of the transfer MISFET Qt₁.

The gate electrode 34 is constituted by a polycrystalline silicon film in which impurity is introduced so as to reduce the resistance value. P type impurity (B) is introduced in the polycrystalline silicon film. The gate electrode 34 is constituted by the polycrystalline silicon film with the p type impurity (B) introduced therein, and connected to the semiconductor area 31 or the gate electrode 27 through the high melting-point metal silicide film 27B interposed therein in order to prevent insertion of a parasitic diode. The gate electrode 34 comprising the polycrystalline silicon film with the p type impurity introduced therein can decrease the threshold voltage of respective load MISFETs Qp₁, Qp₂ in comparison to the case of the n type gate electrode. The decrease of the threshold voltage can decrease the introduction quantity of the impurity introduced in the channel forming area 37A of the respective load MISFETs Qp₁, Qp₂, thereby making the introduction quantity of the impurity more controllable.

When the n type impurity (As or P) is introduced in the gate electrode 34, the ohmic characteristics cannot be deteriorated during the connection to respective gate electrodes 27 of the drive MISFETs Qd₁, Qd₂ or the n type semiconductor area 31.

As a result of the basic study by the inventor, effect has been confirmed in that when the gate electrode 34 is formed in film thickness of about 1000 (A) or more, a depletion layer is formed within the gate electrode 34 (polycrystalline silicon film) by the field effect from the gate electrode 27 of the drive MISFET Qd₁ or Qd₂, thereby the field effect from the gate electrode 27 can be shielded by the gate electrode 34. Consequently, the gate electrode 34 is constituted in the above-mentioned film thickness.

The gate electrode 34 is not limited to the polycrystalline silicon film, but may be constituted by a single layer of a high melting-point metal silicide film or a high melting-point metal film. In this case, the conductivity type of the conductive layer connected to the gate electrode 34 has no relation. Also the gate electrode 34 may be a composite film comprising a polycrystalline silicon film, and a high melting-point metal silicide film or a high melting-point metal film on the polycrystalline silicon film.

Capacitance C₅ using the interlayer insulation film 32 as a dielectric layer is formed between the gate electrode 27 of the drive MISFETs Qd₂, Qd₁ and the gate electrode 34 of the load MISFETs Qp₁, Qp₂.

The capacitance C₅ has effect of increasing the capacitance of the storage node portions N₁, N₂ of the flip-flop circuit, as shown in FIG. 5.

The gate insulation film 35 is constituted by a silicon oxide film deposited by means of CVD.

The channel forming area 37A, as shown in FIG. 8 (plan view in the prescribed manufacturing process) in detail is formed on prescribed upper portion of the gate insulation film 35. The channel forming area 37A is constituted by a polycrystalline silicon film of i type where impurity to reduce the resistance value is not introduced or p type impurity is slightly introduced.

The drain area 37B is formed integrally with one end side of the channel forming area 37A, and is constituted by a p type polycrystalline silicon film in which p type impurity is introduced. The drain area 37B is connected through the connection hole 36 formed on the gate insulation film 35 (Part except for the channel forming area 37A is used as an interlayer insulation film) to gate electrode 27. Since the drain area 37B and the gate electrode 27 are connected through the high melting-point metal silicide layer, the drain area 37B and the gate electrode 27 can be connected in the ohmic connection.

The source area 37C is formed integrally with the other end side of the channel forming area 37A, and is constituted by a p type polycrystalline silicon film in which p type impurity is introduced. The source area 37C is formed integral with the power source voltage wiring Vcc extending in the row direction.

As shown in FIG. 3, the gate electrode 34 of the load MISFET Qp₁ is formed to be overlapped positively with the source area 37C and the drain area 37B. According to such overlapping, capacitance C₃ is provided between gate and source of the load MISFET Qp₁, and capacitance C₁ is provided between gate and drain thereof. Also capacitance C₄ is provided between gate and source of the load MISFET Qp₂, and capacitance C₂ is provided between gate and drain thereof. These capacitances C₁ -C₄ are equivalently connected to the information storage nodes N₁, N₂ thereby the capacitance provided in the information storage nodes can be increased. Consequently, the effect that is obtained is that soft error due to α-ray or the like is not easily generated.

In the SRAM having the memory cells of the CMOS type, the gate electrode 34 of the load MISFET Qp in such a memory cell is provided on the upper side of the gate electrode 27 of the drive MISFET Qd (i.e. it is insulatedly disposed thereover), thereby the field effect from the gate electrode 27 of the drive MISFET Qd can be shielded. Consequently, current amount during operating and current amount during the waiting, or quiescent period respectively, of the load MISFET Qp can be optimized independently.

Gate electrodes of the load MISFET and the drive MISFET may be made independent thereby the degree of freedom in the layout can be increased.

Since the gate electrode of the transfer MISFET can be constituted by employing a material of low resistance having a high melting-point silicide layer, the read/write operation of information can be effected at high speed.

Further, since capacitance provided in the information storage node of the memory cell can be increased, the storage quantity of the information storage member can be increased and the soft error can be prevented.

Next, the manufacturing method of the memory cell of SRAM will be briefly described referring to FIGS. 9 through 15 (main part sectional view shown in each manufacturing process).

First, n⁻ type semiconductor substrate 21 comprising monocrystalline silicon is prepared.

Next, in a memory cell forming area and n-channel MISFET forming area of a peripheral circuit (not shown) respectively, p⁻ type well area 22 is formed on main surface portion of the semiconductor substrate 21.

Between elements of the memory cell, a field insulation film 23 and p type channel stopper area 24 are formed on main surface of the well area 22.

As shown in FIG. 9 between element forming areas of the memory cell, a gate insulation film 25 is formed on main surface of the well area 22. The gate insulation film 25 is constituted by a silicon oxide film formed by oxidizing the main surface of the well area 22. The gate insulation film 25 is formed in film thickness of about 250-350 (A) for example.

As shown in FIG. 10, a connection hole 26 is formed. The connection hole 26 may be formed by partially removing the gate insulation film 25 in portion where a gate electrode 27 is connected directly to main surface of the well area 22.

As shown in FIG. 11, a gate electrode 27, a word line 27 and a reference voltage wiring are formed. The gate electrode 27 is formed by a composite film comprising a polycrystalline silicon film 27A, and a high melting-point metal silicide film 27B stacked on upper side of the silicon film 27A. The polycrystalline film 27A is deposited by means of CVD, and P being n type impurity to reduce the resistance value is introduced therein. The polycrystalline silicon film 27A is formed in film thickness of about 2000-3000 (Å) for example. The high melting-point metal silicide film 27B is deposited by means of sputtering. The high melting-point metal silicide film 27B is formed in film thickness of about 2500-3500 (Å) for example. The polycrystalline silicon film 27A and the high melting-point metal silicide film 27B are subjected to patterning by anisotropy etching of RIE or the like.

As shown in FIG. 12, n type semiconductor area 29 to be used as source area and a part drain area is formed. The semiconductor area 29 can be formed, for example, in that P of about 10¹³ (atoms/cm²) is introduced by ion implantation of energy of about 40-60 (KeV). In the introduction of the impurity, the gate electrode 27 and the field insulation film 23 are mainly used as an impurity introducing mask. Consequently, the semiconductor area 29 can be formed in self matching to the gate electrode 27.

Also as shown in FIG. 12, n⁺ type semiconductor area 28 is formed on main surface portion of the well area 22 where the gate electrode 27 is connected through the connection hole 26. The semiconductor area 28 can be formed in that n type impurity introduced in the polycrystalline silicon film 27A being a lower layer of the gate electrode 27 is subjected to thermal diffusion on the main surface portion of the well area 22. The semiconductor area 28 is formed in the same process as the heat treatment process where, for example, the high melting-point metal silicide film 27B being an upper layer of the gate electrode 27 is activated.

Next, a side wall spacer 30 is formed on a side wall of the gate electrode 27. The side wall spacer 30 can be formed in that a silicon oxide film is deposited by means of CVD so as to cover the gate electrode 27, and anisotropy etching of RIE or the like is applied to the silicon oxide film.

As shown in FIG. 13, n⁺ type semiconductor area 31 to be used as source area and drain area is formed. The semiconductor area 31 can be formed, for example, in that As of about 10¹⁵ -10¹⁶ (atoms/cm²) is introduced by ion implantation of energy of about 40-60 (KeV). In the introduction of the impurity, the gate electrode 27, the field insulation film 23 and the side wall spacer 30 are mainly used as an impurity introducing mask. Consequently, the semiconductor area 31 can be formed in self matching to the side wall spacer 30. Since the semiconductor area 31 is formed, the transfer MISFETs Qt₁, Qt₂ and the drive MISFETs Qd₁, Qd₂ are completed respectively.

Although not shown, p⁺ type semiconductor area being source area and drain area of p-channel MISFET to constitute a peripheral circuit is formed after the process to form the semiconductor area 31.

Next, an interlayer insulation film 32 is formed on the whole surface of the substrate including upper side of the gate electrode 27. The interlayer insulation film 32 is formed by a silicon oxide film deposited by CVD and having fine film property. The interlayer insulation film 32 is formed in film thickness as thin as about 300-1500 (Å) so that the growth of stepped form is relieved and the step coverage of the conductive layer of the upper layer can be improved.

In the connecting portion between the gate electrode 27 and the gate electrode 34, the interlayer insulation film 32 is partially removed and a connection hole 33 is formed.

As shown in FIG. 14, respective gate electrode 34 of the load MISFETs Qp₁, Qp₂ to be connected to the gate electrode 27 through the connect on hole 33 are formed. The gate electrode 34 is constituted by a polycrystalline silicon film deposited by means of CVD. The gate electrode 34 is formed in film thickness as thin as about 1000-1500 (Å) for example. P of about 10¹⁵ -10¹⁶ (atoms/cm²) is introduced into the gate electrode 34 by ion implantation of energy of about 20-40 (KeV). That is, the gate electrode 34 is formed by the polycrystalline silicon film of n type.

Next, a gate insulation film 35 is formed on the whole surface of the substrate so as to cover the gate electrode 34. The gate insulation film 35 is formed, for example by a silicon oxide film deposited by CVD and having fine film property. The gate insulation film 35 is formed in film thickness of about 1000-1500 (Å) for example.

As shown in FIG. 15, on upper side of the gate insulation film 35 respective channel forming areas 37A of the load MISFETs Qp₁, Qp₂, drain area 37B and source area 37C (including the power source voltage wiring are formed in sequence. The channel forming area 37A, the drain area 37B and the source area 37C is constituted by a polycrystalline silicon film deposited by CVD for example and formed in film thickness of about 650-2000 (Å). The drain area 37B and the source area 37C are formed in p type, for example, in that BF₂ of about 10¹⁵ (atoms/cm²) is introduced into the polycrystalline silicon film by ion implantation of energy of about 50-70 (KeV). Since the channel forming area 37A, the drain area 37B and the source area 37C are formed, the load MISFETs Qp₁ and Qp₂ are completed.

Next, an interlayer insulation film 38 is formed on the whole surface of the substrate. The interlayer insulation film 38 is constituted, for example, by a composite film comprising a silicon oxide film deposited by CVD, and a PSG film formed on upper side of the silicon oxide film by CVD. And then a connection hole 39 is formed on the interlayer insulation film 38.

Next, as shown in FIGS. 3 and 4 a data line 40 is formed on upper side of the interlayer insulation film 38 so that connection is effected to respective other semiconductor areas 31 of the transfer MISFETs Qt₁, Qt₂ through the connection hole 39.

These series of the manufacturing processes are applied thereby the memory cell of SRAM of the embodiment is completed.

FIG. 16 shows an example where in the structure of the embodiment of the invention shown in FIG. 3, the conductive layer to constitute gate electrodes of the drive MISFETs Qd₁, Qd₂ and the transfer MISFETs Qt₁, Qt₂ is made structure of three layers, a polycrystalline silicon film, TiN and a high melting-point metal silicide layer in the order from the lower layer.

Since the gate electrode 27 of the drive MISFETs Qd₁, Qd₂ is N type, and the gate electrode 34 of the load MISFETs Qp₁, Qp₂ and the source-drain area 37B, 37C are P type, if both are connected directly, the impurities will be mutually diffused. However, the above-mentioned structure shown in FIG. 16 can prevent the mutual diffusion of the impurities. Further if a barrier layer of TiN is interposed between the polycrystalline silicon film and the high melting-point metal silicide layer, such problem can be prevented that the high melting-point metal passes through the polycrystalline silicon film and enters the gate insulation film under the polycrystalline silicon film thereby the withstanding voltage of the gate insulation film is deteriorated. Of course this structure of the gate electrodes may be applied to other examples of the invention.

FIG. 17 shows an example where the gate electrode of the load MISFETs Qp₁, Qp₂ is provided on upper side of area of source, drain and channel and is a sectional view taken in cutting line XVII--XVII of FIG. 18. Also FIG. 19 is an equivalent circuit diagram of the memory cell shown in FIG. 18. Reference numerals of parts in FIGS. 17-19 are the same as those in FIGS. 3-5. This embodiment is different from the previous embodiment in that the gate electrode of the load MISFETs Qp₁, Qp₂ is provided on upper side of area of source, drain and channel. That is, the area of source, drain and channel of the load MISFETs Qp₁, Qp₂ is formed by the polycrystalline silicon layer being the second layer, and the gate electrode is formed by the polycrystalline silicon layer being the third layer.

Further, area of source and drain of the load MISFETs Qp₁, Qp₂ is an impurity area in which boron is introduced. The boron is introduced using the gate electrode 34 as a mask and then annealed so that the gate electrode and the impurity area are overlapped. It follows that the capacitance due to the overlapping between the gate electrode and the area of source and drain is connected as in C₁ -C₄ of FIG. 19. As a result, capacitance to be added to the information storage nodes can be increased.

FIGS. 20 and 21 are views illustrating the manufacturing method of the memory cell shown in FIGS. 17-19. In similar manner to that in FIGS. 9-14, the polycrystalline silicon films in the first layer and the second layer are formed. However, plane pattern of the polycrystalline silicon film in the second layer is different from that in FIG. 14.

As shown in FIG. 20, the polycrystalline silicon film 37 deposited by CVD for example is formed in film thickness of 650-2000 (Å), and then, as shown in FIG. 21, the gate insulation film 35 is formed in film thickness of about 200-400 (Å) for example. Further, the polycrystalline silicon film 34 is formed in film thickness of 100-1500 (Å) onto the gate insulation film 35 by CVD. The polycrystalline silicon film 34 is subjected to patterning as shown in FIG. 18. And then ion implantation of BF₂ of about 10¹⁵ (atoms/cm²) is effected in energy of about 50-70 (KeV) onto the gate electrode and the area of source and drain of the load MISFETs Qp₁, Qp₂, and annealing of 850°-950° C. is effected thereby the boron implanted into the polycrystalline silicon film 37 is diffused in the lateral direction. Consequently the overlapping capacitance is formed between the area of source and drain and the gate electrode.

The gate electrode is used as the mask in the ion implantation for forming the area of source and drain as above described, thereby the area of source and drain can be formed in self matching to the gate electrode, and further the manufacturing process can be simplified.

An example shown in FIGS. 22 and 23 is nearly similar to the example shown in FIGS. 17-19, but different in the plane pattern of the gate electrode 34 of the load MISFETs Qp₁, Qp₂. FIG. 22 is a sectional view taken in cutting line XXII--XXII of FIG. 23. In this example, the gate electrode 34 of the load MISFETs Qp₁, Qp₂ is widely overlapped with the area of source and drain. The gate electrode 34 is overlapped with the area of source and drain as above described, thereby the capacitance of C1-C4 of FIG. 19 can be increased.

In this case, however, since the gate electrode of the load MISFET cannot be made a mask in the ion implantation for forming the area of source and drain as described in FIG. 21, the manufacturing process number is increased corresponding to this.

FIGS. 24 and 25 show an example where the polycrystalline silicon film in the second layer is used as a reference voltage wiring. FIG. 24 is a sectional view taken in cutting line XXIV--XXIV of FIG. 25.

A reference voltage wiring 42 constituted by the polycrystalline silicon film in the second layer is formed on the gate electrode 27 of the drive MISFETs Qd₁, Qd₂ as shown in FIG. 25. The reference voltage wiring 42 is arranged between the gate electrode 27 of the drive MISFETs Qd₁, Qd₂ and the channel area 37A(i) of the load MISFETs Qp₁, Qp₂, and extends in the parallel direction to the word line.

According to this constitution, the field effect to the load MISFET from the gate electrode 27 of the drive MISFETs Qd₁, Qd₂ can be shielded. Consequently, it can be prevented that the field effect of the gate electrode of the drive MISFETs Qd₁, Qd₂ varies the current amount of the load MISFET in the operating state and the waiting state.

Further, since the reference voltage wiring 42 can be formed on the forming area of the drive MISFETs Qd₁, Qd₂, the memory cell area can be made small. 

What is claimed is:
 1. A semiconductor memory device comprising:a semiconductor substrate having a main surface; memory cells of a static random access memory, each of said, memory cells including first and second inverter circuits which are cross-coupled, said first inverter circuit including a first drive MISFET and a first load MISFET coupled in series, and said second inverter circuit including a second drive MISFET and a second load MISFET coupled in series,wherein each of said first and second drive MISFETs includes source and drain regions which are of n-type conductivity and formed in said semiconductor substrate, a gate insulating film formed over said main surface, and a gate electrode formed over said gate insulating film; first polycrystalline silicon films of n-type conductivity formed over said main surface and being electrically connected to the drain regions of said first and second drive MISFETs of said each memory cell; and a first insulating film formed over said gate electrodes of said first and second drive MISFETs of each memory cell so as to cover said main surface,wherein said first and second load MISFETs are formed over said first insulating film, each load MISFET of a memory cell including a drain region, a source region, and a channel forming region, the drain, source and channel forming regions of said each load MISFET are formed in a respective one of plural second polycrystalline silicon films, and said each load MISFET further including a gate electrode and having a gate insulating film formed between said second polycrystalline silicon film associated therewith and the gate electrode thereof, and wherein, in said each memory cell, the drain regions of said first and second load MISFETs have a p-type conductivity and are electrically connected to the drain regions of said first and second drive MISFETs through separate ones of said first polycrystalline silicon films, respectively.
 2. A semiconductor memory device comprising:a semiconductor substrate having a main surface; memory cells of a static random access memory, each of said memory cells including first and second inverter circuits which are cross-coupled, said first inverter circuit including a first drive MISFET and a first load MISFET coupled in series, and said second inverter circuit including a second drive MISFET and a second load MISFET coupled in series,wherein each of said first and second drive MISFETs includes source and drain regions which are of n-type conductivity and formed in said semiconductor substrate, a gate insulating film formed over said main surface, and a gate electrode formed over said gate insulating film; first polycrystalline silicon films formed over said main surface and being electrically connected to the drain regions of said first and second drive MISFETs of said each memory cell; and a first insulating film formed over said gate electrodes of said first and second drive MISFETs of each memory cell so as to cover said main surface,wherein said first and second load MISFETs are formed over said first insulating film, each load MISFET of a memory cell including a drain region, a source region, and a channel forming region, the drain, source and channel forming regions of said each load MISFET are formed in a respective one of plural second polycrystalline silicon films, and said each load MISFET further including a gate electrode and having a gate insulating film formed between said second polycrystalline silicon film associated therewith and the gate electrode thereof, and wherein, in said each memory cell, the drain regions of said first and second load MISFETs have a p-type conductivity and are electrically connected to the drain regions of said first and second drive MISFETs and to separate ones of said first polycrystalline silicon films, respectively.
 3. A semiconductor memory device comprising:a semiconductor substrate having a main surface; a memory cell including a first and a second inverter circuit, said first inverter circuit including a first n-channel drive MISFET and a first p-channel load MISFET, and said second inverter circuit including a second n-channel drive MISFET and a second p-channel load MISFET, each drive MISFET and each load MISFET having a source region, a drain region and a gate electrode,wherein the drain region of said first drive MISFET is electrically connected to the drain region of said first load MISFET, the gate electrode of said second load MISFET, and the gate electrode of said second drive MISFET, wherein the drain region of said second drive MISFET is electrically connected to the drain region of said second load MISFET, the gate electrode of said first load MISFET, and the gate electrode of said first drive MISFET, and wherein the source region and drain region of each of said first and second drive MISFETs are formed in said semiconductor substrate and have n-type conductivity, the gate electrode of said each drive MISFET being formed over a gate insulating film thereof formed over said main surface; a first insulating film formed over the gate electrodes of said first and second drive MISFETs so as to cover said main surface,wherein the drain region, the source region, and a channel forming region of a respective load MISFET are together provided in a first polycrystalline silicon film formed over said first insulating film, the gate electrode of said respective load MISFET being formed over said first insulating film, and wherein said each load MISFET has a gate insulating film formed between a respective said first polycrystalline silicon film associated therewith and the gate electrode thereof, the drain region of said each load MISFET having a p-type conductivity; and a second polycrystalline silicon film having an n-type conductivity being formed over said main surface and being electrically connected to the drain region of a respective drive MISFET,wherein said first polycrystalline silicon film is extended over said second polycrystalline silicon film and is electrically connected to said second polycrystalline silicon film such that the drain region of said each load MISFET is electrically connected to a respective said second polycrystalline silicon film and such that the drain region of a load MISFET of a respective one of said first and second inverter circuits is electrically connected to the drain region of a drive MISFET of the same inverter circuit through said respective second polycrystalline silicon film.
 4. A semiconductor memory device according to claim 3,wherein a respective said second polycrystalline silicon film is integrally formed with the gate electrode of said respective load MISFET, and wherein said second polycrystalline silicon film is formed over said first insulating film.
 5. A semiconductor memory device according to claim 4,wherein said respective second polycrystalline silicon film is electrically connected to the gate electrode of said respective drive MISFET, and wherein the gate electrode of said respective drive MISFET has an n-type conductivity.
 6. A semiconductor memory device according to claim 3, wherein the gate electrode of said respective load MISFET is formed over said respective first polycrystalline silicon film.
 7. A semiconductor memory device according to claim 6, wherein a respective said second polycrystalline silicon film is integrally formed with the gate electrode of said respective drive MISFET.
 8. A semiconductor memory device according to claim 3, wherein a respective said second polycrystalline silicon film is integrally formed with the gate electrode of said respective drive MISFET.
 9. A semiconductor memory device according to claim 6, wherein said respective first polycrystalline silicon film is electrically connected to said respective second polycrystalline silicon film through a silicide film formed on said respective second polycrystalline silicon film.
 10. A semiconductor memory device according to claim 3, wherein said respective first polycrystalline silicon film is electrically connected to said respective second polycrystalline silicon film through a silicide film formed on said respective second polycrystalline silicon film.
 11. A semiconductor memory device according to claim 3, wherein said semiconductor substrate is formed of monocrystalline silicon.
 12. A semiconductor memory device comprising:a semiconductor substrate having a main surface; a memory cell including a first inverter circuit, a second inverter circuit, a first n-channel transfer MISFET, and a second n-channel transfer MISFET, said first inverter circuit including a first n-channel drive MISFET and a first p-channel load MISFET, and said second inverter circuit including a second n-channel drive MISFET and a second p-channel load MISFET, each of the drive, load and transfer MISFETs having a source region, a drain region and a gate electrode,wherein the drain region of said first drive MISFET is electrically connected to the drain region of said first load MISFET, the gate electrode of said second load MISFET, the gate electrode of said second drive MISFET, and one of the source region and drain region of said first transfer MISFET, wherein the drain region of said second drive MISFET is electrically connected to the drain region of said second load MISFET, the gate electrode of said first load MISFET, the gate electrode of said first drive MISFET, and one of the source region and drain region of said second transfer MISFET, wherein the source region and drain region of each of said first and second transfer MISFETs are formed in said semiconductor substrate and have n-type conductivity, wherein the source region and drain region of each of said first and second drive MISFETs are formed in said semiconductor substrate and have n-type conductivity, a gate insulating film of each of said first and second drive MISFETs being formed over said main surface, and the gate electrode of each of said first and second drive MISFETs being formed over the gate insulating film thereof; and a first insulating film formed over a conductive film so as to cover said main surface,wherein the drain region, the source region, and a channel forming region of each load MISFET are together provided in a respective polycrystalline silicon film formed over said first insulating film, the gate electrode of said each load MISFET being formed over said first insulating film, wherein said each load MISFET has a gate insulating film thereof formed between said respective polycrystalline silicon film and the gate electrode thereof, wherein the gate electrode of each drive MISFET is formed of a polycrystalline silicon film and has n-type conductivity, and the gate electrode of said each load MISFET is formed of a polycrystalline silicon film and has n-type conductivity, wherein the drain region of said first drive MISFET is electrically connected to one of the source region and drain region of said first transfer MISFET, the gate electrode of said second drive MISFET, and the gate electrode of said second load MISFET so as to constitute a first storage node of said memory cell, and wherein the drain region of said first load MISFET has a p-type conductivity and is electrically connected to said first storage node such that said drain region of said first load MISFET is electrically connected to the gate electrode of one of said second drive MISFET and said second load MISFET.
 13. A semiconductor memory device according to claim 12,wherein the channel forming region of said each load MISFET is formed over the gate electrode thereof, and wherein the drain region of said first load MISFET is electrically connected to the gate electrode of said second load MISFET.
 14. A semiconductor memory device according to claim 13,wherein the gate electrode of said second load MISFET is electrically connected to the gate electrode of said second drive MISFET, and wherein the gate electrode of said second drive MISFET is electrically connected to the drain region of said first drive MISFET and to one of the source region and drain region of said first transfer MISFET.
 15. A semiconductor memory device according to claim 12, wherein the gate electrode of said each load MISFET is formed over the channel forming region thereof.
 16. A semiconductor memory device according to claim 15,wherein the gate electrode of said second load MISFET is electrically connected to the gate electrode of said second drive MISFET, and wherein the gate electrode of said second drive MISFET is electrically connected to the drain region of said first drive MISFET and to one of the source region and drain region of said first transfer MISFET.
 17. A semiconductor memory device according to claim 12,wherein said drain region of said second drive MISFET is electrically connected to one of the source region and drain region of said second transfer MISFET, the gate electrode of said first drive MISFET, and the gate electrode of said first load MISFET so as to constitute a second storage node of said memory cell, wherein the drain region of said second load MISFET has a p-type conductivity and is electrically connected to said second storage node such that the drain region of said second load MISFET is electrically connected to the gate electrode of one of said first drive MISFET and said first load MISFET, and wherein said semiconductor substrate is formed of monocrystalline silicon.
 18. A semiconductor memory device comprising:a semiconductor substrate having a main surface; a memory cell including a first inverter circuit, a second inverter circuit, a first n-channel transfer MISFET, and a second n-channel transfer MISFET, said first inverter circuit including a first n-channel drive MISFET and a first p-channel load MISFET, and said second inverter circuit including a second n-channel drive MISFET and a second p-channel load MISFET, each of the drive, load and transfer MISFETs having a source region, a drain region and a gate electrode,wherein the drain region of said first drive MISFET is electrically connected to the drain region of said first load MISFET, the gate electrode of said second load MISFET, the gate electrode of said second drive MISFET, and one of the source region and drain region of said first transfer MISFET, wherein the drain region of said second drive MISFET is electrically connected to the drain region of said second load MISFET, the gate electrode of said first load MISFET, the gate electrode of said first drive MISFET, and one of the source region and drain region of said second transfer MISFET, wherein the source region and drain region of each of said first and second transfer MISFETs are formed in said semiconductor substrate and have n-type conductivity, wherein the source region and drain region of each of said first and second drive MISFETs are formed in said semiconductor substrate and have n-type conductivity, a gate insulating film of each of said first and second drive MISFETs being formed over said main surface, and the gate electrode of each of said first and second drive MISFETs being formed over the gate insulating film thereof; and a first insulating film formed over a conductive film so as to cover said main surface,wherein the drain region, the source region, and a channel forming region of each load MISFET are together provided in a respective polycrystalline silicon film formed over said first insulating film, the gate electrode of each load MISFET being formed over said first insulating film, wherein said load MISFET has a gate insulating film thereof formed between said respective polycrystalline silicon film and the gate electrode thereof, the gate electrode of each drive MISFET being formed of a polycrystalline silicon film and having an n-type conductivity, the gate electrode of said each load MISFET being formed of a polycrystalline silicon film and having a p-type conductivity, and the drain region of said first load MISFET having a p-type conductivity, wherein the drain region of said first drive MISFET is electrically connected to said one of the source region and drain region of said first transfer MISFET, and the gate electrode of said second drive MISFET so as to constitute a first storage node of said memory cell, one of the drain region of said first load MISFET and gate electrode of said second load MISFET being electrically connected to said first storage node such that said one of the drain region of said first load MISFET and gate electrode of said second load MISFET is electrically connected to the gate electrode of said second drive MISFET.
 19. A semiconductor memory device according to claim 18, wherein said one of the drain region of said first load MISFET and gate electrode of said second load MISFET is electrically connected to the gate electrode of said second drive MISFET through a silicide film, constituting a respective said conductive film, formed on said gate electrode of said second drive MISFET.
 20. A semiconductor memory device according to claim 18,wherein the drain region of said second load MISFET has a p-type conductivity, wherein the drain region of said second drive MISFET is electrically connected to said one of the source region and drain region of said second transfer MISFET and the gate electrode of said first drive MISFET so as to constitute a second storage node of said memory cell, wherein one of the drain region of said second load MISFET and gate electrode of said first load MISFET is electrically connected to said second storage node such that said one of the drain region of said second load MISFET and gate electrode of said first load MISFET is electrically connected to the gate electrode of said first drive MISFET, and wherein said semiconductor substrate is formed of monocrystalline silicon.
 21. A semiconductor memory device according to claim 20, wherein the gate electrode of said each load MISFET is formed over the channel forming region thereof.
 22. A semiconductor memory device according to claim 20, wherein the gate electrode of said each load MISFET is formed below the channel forming region thereof.
 23. A semiconductor memory device comprising:a semiconductor substrate of monocrystalline silicon having a main surface; a memory cell including a first and a second inverter circuit, said first inverter circuit including a first n-channel drive MISFET and a first p-channel load MISFET, and said second inverter circuit including a second n-channel drive MISFET and a second p-channel load MISFET, each drive MISFET and each load MISFET having a source region, a drain region and a gate electrode,wherein the drain region of said first drive MISFET is electrically connected to the drain region of said first load MISFET, the gate electrode of said second load MISFET, and the gate electrode of said second drive MISFET, wherein the drain region of said second drive MISFET is electrically connected to the drain region of said second load MISFET, the gate electrode of said first load MISFET, and the gate electrode of said first drive MISFET, wherein the source region and drain region of each of said first and second drive MISFETs are formed in said semiconductor substrate and have n-type conductivity, the gate electrode of said each drive MISFET being formed over a gate insulating film thereof formed over said main surface; a first insulating film formed over the gate electrodes of said first and second drive MISFETs so as to cover said main surface,wherein the drain region, the source region, and a channel forming region of a respective load MISFET are together provided in a respective first polycrystalline silicon film formed over said first insulating film, the gate electrode of said each load MISFET being formed over said first insulating film, and wherein said each load MISFET has a gate insulating film formed between said first respective polycrystalline silicon film and the gate electrode thereof, the drain region of said each load MISFET having a p-type conductivity; and a second polycrystalline silicon film having n-type conductivity formed over said main surface and formed of a different level layer of polycrystalline silicon film from that of said first polycrystalline silicon film,wherein said respective first polycrystalline silicon film is electrically connected to a respective said second polycrystalline silicon film such that said respective second polycrystalline silicon film is electrically connected to the drain region of a respective drive MISFET and such that said drain region of said load MISFET of one of said first and second inverter circuits is electrically connected to said drain region of the drive MISFET of the same inverter circuit through said second polycrystalline silicon film.
 24. A semiconductor memory device according to claim 23, wherein a respective said second polycrystalline silicon film is integrally formed with the gate electrode of said respective load MISFET.
 25. A semiconductor memory device according to claim 24,wherein a respective said second polycrystalline silicon film is electrically connected to the gate electrode of said respective drive MISFET, and wherein the gate electrode of said respective drive MISFET has n-type conductivity.
 26. A semiconductor memory device according to claim 23, wherein a respective said second polycrystalline silicon film is integrally formed with the gate electrode of said respective drive MISFET.
 27. A semiconductor memory device according to claim 26, wherein said respective first polycrystalline silicon film is formed over the gate electrode of said respective load MISFET.
 28. A semiconductor memory device according to claim 26, wherein said respective first polycrystalline silicon film is formed below the gate electrode of said respective load MISFET.
 29. A semiconductor memory device according to claim 19, wherein the gate electrodes of said drive MISFETs are integrally formed with said first polycrystalline silicon films in such a manner that the gate electrodes of said first and second drive MISFETs in a respective memory cell are electrically connected to the drain regions of said second and first load MISFETs, respectively.
 30. A semiconductor memory device comprising:a semiconductor substrate having a main surface; memory cells of a static random access memory, each of said memory cells including first and second inverter circuits which are cross-coupled, said first inverter circuit including a first drive MISFET and a first load MISFET coupled in series, and said second inverter circuit including a second drive MISFET and a second load MISFET coupled in series, wherein each of said first and second drive MISFETs includes source and drain regions which are of n-type conductivity and formed in said semiconductor substrate, a gate insulating film formed over said main surface, and a gate electrode formed over said gate insulating film; first polycrystalline silicon films of n-type conductivity formed over said main surface and being electrically connected to the drain regions of said first and second drive MISFETs of said each memory cell; and a first insulating film formed over said gate electrodes of said first and second drive MISFETs of each memory cell so as to cover said main surface, wherein said first and second load MISFETs are formed over said first insulating film, each load MISFET of a memory cell including a drain region, a source region, and a channel forming region, the drain, source and channel forming regions of said each load MISFET are formed in a respective one of plural second polycrystalline silicon films, and said each load MISFET further including a gate electrode and having a gate insulating film formed between said second polycrystalline silicon film associated therewith and the gate electrode thereof, and wherein, in said each memory cell, the drain regions of said first and second load MISFETs have a p-type conductivity and are electrically connected to the drain regions of said first and second drive MISFETs and to separate ones of said first polycrystalline silicon films, respectively. 